This invention relates generally to automatic test equipment for electronics (ATE) and, more particularly, to the synthesis of low-noise, high frequency waveforms for testing microwave and RF circuitry.
Significant improvements in the accuracy of high-frequency devices used in consumer products such as cellular telephones, pagers, and wireless personal data assistants (PDAs) have created a need for more accurate testing of these devices. ATE systems generally include one or more microwave synthesizer for testing microwave devices. In one typical testing scenario, a microwave synthesizer within the tester supplies a signal directly to the DUT. The DUT provides a response, which the tester measures and tests. In another testing scenario, a tester receives a microwave signal (e.g., 900 MHz) from a device under test (DUT). The tester mixes this signal with the output of one of its microwave synthesizers to generate an intermediate frequency signal (e.g., 10 MHz). The tester then samples the intermediate frequency signal to ascertain its characteristics. If the characteristics are within predetermined limits, the test passes. Otherwise, the test fails.
One common testing technique is to compute a power spectrum of the intermediate frequency signal derived from the device under test. A power spectrum reveals meaningful information about the DUT as well as phase noise. To accurately test the phase noise of the device under test, it is essential that the synthesizer""s phase noise be small compared with that of the DUT. If the synthesizer""s phase noise is large compared with that of the DUT, the DUT""s phase noise becomes lost in the synthesizer""s phase noise, and it becomes impossible to tell whether the DUT meets its phase noise specification. As devices are continually improved to deliver lower and lower phase noise, microwave synthesizers must correspondingly be improved if testing is to remain accurate.
FIG. 1 illustrates a conventional microwave synthesizer 100, which operates as follows. A narrow-band synthesizer 112 generates an output signal that can be varied over a relatively narrow range, e.g., a 200 MHz range between 800 MHz and 1 GHz. Simultaneously, a wide-band synthesizer 122 generates an output signal that can be varied over a relatively wide frequency range, e.g., a 2 GHz range between 4.4 GHz and 6.2 GHz. Simultaneously, a comb generator 116 produces a series of harmonically spaced tones, or xe2x80x9ccombs,xe2x80x9d e.g., at 200 MHz tone spacing. The output of the narrow-band synthesizer 112, the wide-band synthesizer 114, and the comb generator 116 are respectively fed to a narrow-band input 152, a wide-band input 154, and a comb input 156 of a drift-cancel loop 150.
Within the drift-cancel loop 150, a power splitter 130 divides the output of the wide-band synthesizer 122 into first and second circuit paths. Amplifiers 132 and 134 boost the levels of signals along the respective paths. A first mixer 138 combines the output of the amplifier 132 with the output of the comb generator 116, to produce a different pair of sum and difference tones for each tone produced by the comb generator 116. By appropriately tuning the frequency of the wide-band synthesizer 122, one of the sum or difference tones from the mixer 138 can be made to equal a target frequency, FK. For normal operation, the inputs to the drift-cancel loop 150 are always adjusted to produce a tone at the output of the mixer 138 that equals FK.
A first band-pass filter 142 filters the output of the mixer 138. The first band-pass filter 142 has a center frequency at FK, and has a narrow bandwidth for passing only the mixing product at FK and substantially rejecting all other frequency components. The output of the first band-pass filter 142 is passed to a second mixer 146, which combines the output of the first band-pass filter 142 with the output of the narrow-band synthesizer 112, thus producing another pair of sum and difference tones. These sum and difference tones are passed to a second band-pass filter 144, which generally rejects the sum tone and transmits the difference tone to its output.
The transmitted tone is passed to a third mixer 140. The third mixer 140 combines the transmitted tone with the output of the amplifier 134 to produce yet another pair of sum and difference tones. A low-pass filter 148 blocks the sum tone and transmits the difference tone to the output of the synthesizer 100. The output may be coupled to additional stages (not shown), for selectively multiplying the frequency and adjusting the amplitude of the output signal.
The output frequency of the synthesizer 100 is adjustable in two ways. First, the wide-band synthesizer 122 can be adjusted to vary the overall output frequency in large increments. Second, the narrow-band synthesizer 112 can be adjusted to vary the overall output frequency in small increments. The narrow band synthesizer 122 generally operates via direct digital synthesis (DDS) to produce a nearly continuous range of output frequencies. The frequency range of the narrow-band synthesizer 112 preferably equals or exceeds the spacing of consecutive combs produced by the comb generator 116, to allow the narrow-band synthesizer to fully tune between adjacent combs. With this arrangement, the wide-band synthesizer 122 effects gross frequency changes, whereas the narrow-band synthesizer 122 effects fine frequency changes. The combination allows the frequency of the synthesizer 100 to be adjusted over a wide range with high precision.
As is known, the wide-band synthesizer 122 tends to produce significant amounts of phase noise. This phase noise is greatly reduced, however, by the action of the drift-cancel loop 150. Owing to the summing and differencing actions of the mixers 138, 140, and 146, the frequency of the wide-band synthesizer 122 is made to cancel from the output of the synthesizer 100. Along with the frequency of the wide-band synthesizer 122, much of its phase noise is made to cancel as well.
In more elaborate implementations, a delay circuit 136 is placed between the second amplifier 134 and the third mixer 140. The delay circuit 136 causes the inputs of the third mixer 140 to convey signals that represent the output of the wide-band synthesizer 122 at corresponding instants of time. By delaying the signal conveyed along the second circuit path to match the delay incurred by the signal along the first circuit path, a great deal of phase noise is canceled by making corresponding phase perturbations common to both inputs of the mixer 140. Because the low-pass filter 148 passes only the difference of input frequencies produced by the mixer 140, noise that is common to both inputs of the mixer 140 is cancelled out.
Even with the addition of the delay circuit 136, the synthesizer 100 still fails to reject some of the phase noise of the wide-band synthesizer 122. Low frequency, or xe2x80x9cclose-in,xe2x80x9d phase noise (less than 1 MHz offset) of the wide-band synthesizer largely cancels out, whereas high frequency, xe2x80x9cfar-out,xe2x80x9d phase noise (above 1 MHz offset) generally does not. In implementations that tightly control the phase noise of the narrow-band synthesizer 112 and the comb generator 116, the overall far-out phase noise of the microwave synthesizer 100 tends to be dominated by the unreduced, far-out phase noise of the wide-band synthesizer 122.
With the foregoing background in mind, it is an object of the invention to reduce the far-out phase noise of signals produced by microwave synthesizers in automatic test equipment.
To achieve the foregoing object, as well as other objectives and advantages, a microwave synthesizer according to the invention includes a drift-cancel loop having a narrow-band input, a low-frequency comb input, a wide-band input, and an output for providing an adjustable-frequency output signal. A narrow-band synthesizer is coupled to the narrow-band input, and a comb generator is coupled to the low-frequency comb input. Instead of using a wide-band synthesizer to drive the wide-band input, as conventional topologies have done, the instant invention employs a low noise, high frequency oscillator. The output of the oscillator is mixed with the output of the comb generator to produce low-noise, high frequency combs. The low-noise, high frequency combs are then used to drive the wide-band input of the drift-cancel loop. Replacing the wide-band synthesizer with high frequency combs can significantly reduce the far-out phase noise of the synthesizer as compared with conventional designs.